Converting An OD Pickoff To An ID Pickoff
It's easy to make equipment purchasing decisions when the customer commits to a fixed number of parts, but it's something else entirely when there is no such commitment and the job could go away at any time.
It’s easy to make equipment purchasing decisions when the customer commits to a fixed number of parts, but it’s something else entirely when there is no such commitment and the job could go away at any time. That was the problem facing Manth-Brownell, Inc. (Kirkville, New York), a producer of precision turned parts for the electronics, automotive, medical equipment, appliance and other industries. The shop wanted to run a brass part on one of its six-spindle automatics. The part has threads on its OD and is trepanned on the back end, and the shop was concerned that the machine’s OD pickoff attachment would damage the threads in the process of presenting the cutoff end of the part for machining.
The shop considered the alternatives. One possibility was to do the back end operations on secondary machines, but the shop preferred to produce the part complete on the multi-spindle machine for the cost and quality benefits. A second possibility was to replace the existing OD pickoff attachment with an ID pickoff attachment available from the machine’s manufacturer that would enable the machine to grab the part on its ID. However, that would involve a $36,000 investment in the attachment itself, a $7,650 kit to fit it to the machine and 8 hours of setup time to install it. Normally, Manth-Brownell would have invested in the changes needed to secure the job, but in this case, the shop had no idea how long the job would last and whether the investment could be amortized over the length of the run. Nor did it know when the next job requiring ID chucking might come along to help justify the investment. A third possibility was for the shop to build the ID pickoff attachment in-house, but the shop wasn’t sure it could handle the task—or if that was something it wanted to do.
While the shop was weighing its options, Wes Skinner, its president, attended a committee meeting of the Precision Machined Products Association (PMPA) and discussed the problem over dinner with Cal Macomber, director of sales & marketing, workholding division, and Neal Des Ruisseaux, manager of the engineering department, for Hardinge Inc. (Elmira, New York) and challenged them to come up with a solution. The company had been purchasing collets from Hardinge for 40 years.
Rising to the challenge, Hardinge engineered an expanding collet to fit the multi-spindle machine’s existing OD pickoff spindle. With the expanding collet, changeover from a standard OD gripping setup to an ID gripping setup becomes a matter of simply changing the collet instead of having to change the entire spindle unit. The Hardinge expanding collet not only costs substantially less than an ID spindle unit, but it also takes less than 15 minutes to install, set up and adjust for proper ejection and clearance.
To remove the expanding collet from the multi-spindle machine’s OD pickoff attachment, the operator pushes in the pin for the retaining ring around the pickoff, rotates it to locate the key, removes the key from the keyway and rotates the collet approximately one-third of a turn and pulls it out. To install the expanding collet, the operator reverses the procedure and resets the collet to the length of the part.
The expanding collet has true parallel gripping with high gripping force and a wide gripping range. A built-in ejector system provides automated parts removal for faster throughput. It fits standard OD pickoff spindles for Euroturn, Gildemeister and Tornos Deco machines and can be made to hold workpieces of a specific size or shape.
Manth-Brownell is currently running the brass part ten shifts per week. The shop has produced 1 million parts using the first expanding collet prototype furnished by Hardinge, and another 750,000 parts have been made with a second collet, without any problems. Actually, the shop had expected some chip-buildup problems, but Hardinge designed openings around the assembly that facilitate flushing of the chips. The machine operator uses shop air to blow out the collet once in a while, but the shop has never had to take one apart to clear packed chips.
The shop reports that with the expanding collet, there is little problem holding tolerances on machined features as the machine goes from a cold start to normal operating temperature. Mr. Skinner attributes the collet’s indifference to temperature to its design and to the fact that there is less friction than would be present in a larger assembly. The shop is also pleased with the adaptability of the expanding collets. “Any screw machine shop in North America could easily make the alterations needed to adapt the collet to another job,” Mr. Skinner insists.
Mr. Skinner speculates that other shops also run into problems gripping on the OD—distorting the part, messing up threads, creating cosmetic problems or some other issue—and don’t even think about gripping the part on the ID because of the additional costs. Once such shops hear about the relatively low cost of the expanding collet and the speed and ease with which it can be installed, he expects that more of them will do more gripping on the ID.
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